Metal coating process

ABSTRACT

A METHOD OF CONTROLLING THE WEIGHT OF CHROMIUM-CONTAINING CONVERSION COATING FORMED ON A METAL SURFACE BY A FLUORIDE ACCELERATED AQUEOUS CHROMATING BATH WHEREIN A FIRST FLUORIDE ACCELERATED AQUEOUS CHROMATING SOLUTION IS CONTACTED WITH A REFERENCE ELECTRODE AND A FLUORIDE SENSITIVE ELECTRODE OF THE POTENTIOMETRIC TYPE AND THE TOTAL POTENTIAL BETWEEN THESE ELECTRODES IN THE SOLUTION IS MEASURED. THESE ELECTRODES ARE THEN USED TO CONTACT A SECOND FLUORIDE ACCELERATED AQUEOUS CHROMATING SOLUTION, THE COMPOSITION OF WHICH IS KNOWN, AS WELL AS THE WEIGHT OF CHROMIUM CONTAINING CONVERSION COATING THAT WILL BE FORMED BY IT AND THE TOTAL POTENTIAL BETWEEN THE TWO ELECTRODES IN THE SECOND SOLUTION IS MEASURED. THE DIFFERENCE BETWEEN THESE TWO POTENTIAL MEASUREMENTS IS DETERMINED AND THIS DIFFERENCE IS MAINTAINED AT A VALUE WHICH WILL CONSISTENTLY PRODUCE THE DESIRED WEIGHT OF COATING BY THE FIRST CHROMATING SOLUTION BY THE SELECTED ADDITION OF SOLUTION COMPONENTS TO THIS CHROMATION SOLUTION.

g- 1, 1972 w. s. RUSSELL 3,681,207

METAL COATING PROCESS Filed Dec. 28, 1970 a? LL 3 24 2 BE 9 22 :1 i 5 20 O A MV United States Patent Office US. 'Cl. 204-1 T 3 Claims ABSTRACT OF THE DISCLOSURE A method of controlling the weight of ,chromium'containing conversion coatingformed on a metal surface by a fluorideaccelerated aqueous chromating bath wherein a first fluoride accelerated aqueous chromating solution is contacted with a reference electrode and a fluoride sensitive electrode of the potentiometric typeand the total potential between these electrodes in the solution is measured. These electrodes are then used to contactga second fluoride accelerated aqueous chromating solution, the composition of which is known, as well as the weight of chromium containing conversion coating that will be formed by it and the total potential between the two elec-- trodes in the second solution is measured. Thedilference between these two potential measurements is determined and this difference is maintained at a value which will consistently produce the desired weight of coating by the first chromating solution by the selected addition of solution components to this chromating solution.

v This invention relates to a method for operating fluoride-accelerated aqueous conversion coating solutions and more particularly it relates to an improved method for controlling the weight of chromium-containing conversion coating formed on a metal surface which is treated by a fluoride accelerated aqueous chromating bath.

In' the application of conversion coatings to metal surfaces, 'it is well known that the character of the coating produced, e.g., thequality and coating weight ofv the coating, depends upon the compositionlof the conversion coating solution which is used. Thus, variations in the relative amounts of the components of the coating solution will have a marked effect on the coating formed on the metal surfaces which are treated. It is, therefore, desirable V in the use of such conversion coating solutions that the amounts of the components in the coating bath be controlled and maintained at the optimum level so as consistently to produce a coating which not only has the desired coating weight but which will also be of the desired high quality. v V

In the case of fluoride accelerated conversion'coating solutions, such as those containing chromate and/ or phosphate, it has generally been relatively easy to maintain the desired concentration of the chromate and/ or phosphate coating components. Typically, this has been done by conductivity and/ or titration techniques, carriedout either manually or automatically, or by a combination of both. With the fluoride accelerator in these solutions, however, difliculties have frequently been encountered in controlling and maintaining the fluoride content of the solutions at the optimum, desired level. v

In general, these difliculties have-been caused by the fact that the nature of the conversion coatings formed, and in particular the weight of the coating, is not directly dependent upon the total fluoride content of the coating bath. Thus, although it is known that there is a relation,- ship between the fluoride of the bath and the coating weight produced, the exact nature of this relationship has not been clear. Accordingly, previous attempts to control resulted in approximations, based on average fluoride requirements, rather than on the actual amounts necessary to continually obtain, from the bath, the desired, optimum coating weights. Moreover, these prior control methods have often involved time consuming, manual, analytical techniques, which have not been completely suitable for use with commercial, high speed coating processes."

Even where automatic techniques have been employed, it

has generally been found that these involve comparisons" provide an improved process for the operation of a fluoride-accelerated aqueous conversion coating solution, whereby the weight of coating produced on the metal surface treated is accurately controlled.

A further object of the present invention is to provide an improved method for controlling the weight of C01]: version coating produced on metal surface by a fluorideaccelerated aqueous conversion coating solution.

Another object of the present invention is to provide an improved process as described above, wherein repeated standardizations of the control equipment are not necessary.

These and other objects of the present invention will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a method for controlling the weight ofconversion coating formed on a metal surface by a fluorideaccelerated aqueous conversion coating solution which comprises contacting a reference electrode and a fluoridesensitive electrode of the potentiometric type with a first fluoride accelerated aqueous conversion coating solution, measuring the total potential between said two electrodes in said solution, contacting said two electrodes with a second fluoride-accelerated aqueous conversion coating solu tion, the composition of which and the weight of conversion coating formed by which is known, measuring'the total potential between said two electrodes in said second solution, determining the difference between said two potential measurements and maintaining this difference at a value which will consistently produce the desired weight of conversion coating by said first solution by the selected addition'of solution components to said first conversion coating solution. By operating in this manner, the weight of conversion coating produced on the metal surface treated is consistently controlled and maintained at the desired, optimum level, for the particular operation involved, by means of a relatively simple, substantially automatic measurement and control, without the need for repeated standardizations of the measuring equipment.

More specifically, in the practice of the present inven- V tion,'the control method described is applicable to a wide "fluoride content in such'coating solutions have frequently variety of fluoride accelerated, aqueous conversion coating solutions. Suitable fluoride-accelerator conversion coating solutions with which the method of the present invention may be used include fluoride accelerated phosphate conversion coating solutions, such as those containing zinc, iron, manganese, alkali metal, and/or alkaline earth metal phosphates, phosphoric acid, and the like, fluorideaccelerated chromate conversion coating solutions, such as those containing chromic acid and/or other sources of hexavalent chromium, as well as those containing both phosphates and hexavalent chromium. In addition to the phosphate and/or hexavalent chromium and fluoride accelerator, such conversion coating solutions may also contain other components such as arsenates, complex cyanides, various organic and inorganic oxidizing agents,

nickel, tungstates, molybdates, and the like, as are well Patented Aug. 1',- 1972' known to those in the art. Such treating solutions may be used for forming conversion coatings on a variety of metal surfaces, such as zinc, aluminum and ferrous metal surfaces, as well as surfaces of the various zinc, aluminum and ferrous metal alloys. The present method has been found to be particularly suitable for controlling the weight of coating produced on aluminum and aluminum alloy surfaces by a fluoride accelerated conversion coating solution containing both phosphate and hexavalent chromium, typically as a mixture of phosphoric acid and chromic acid, as is described in US. Pat. 2,928,763, issued Mar. 15, 1960. Accordingly, hereinafter, particular reference will be made to such aqueous conversion coating solutions although it is to be understood that these are merely exemplary of the solutions which may be utilized.

In carrying out the method of the present invention, the metal surface to be treated is contacted with the aqueous conversion coating solutions for a period sufficient to form the desired coating on the metal surface. Typical of a preferred solution for coating aluminum is an aqueous solution containing from about to 150 grams per liter P0 ions, from about 2.5 to 62 grams per liter, CrO from about 1 to 55 grams per liter of aluminum ions and from about 2.5 to 123 grams per liter fluoride ions. The contact between the metal surface to be treated and the aqueous conversion coating solution may be effected using a variety of application techniques, such as spraying, flooding, immersion, brushing, roll coating, and the like. In many instances, the preferred application techniques are spray or immersion. Using these techniques, the conversion coating solution may be applied to metal in the form of sheet or strip, as well as to formed metal articles, such as, for example in the case of aluminum, formed aluminum cans.

In using the method of the present invention to effect a control of the coating weights produced by these solutions on the metal surfaces which are treated, these coating solutions are contacted with a reference electrode and a fluoride sensitive electrode of the potentiometric type and the total potential between these electrodes in this solution is measured. This contact of the electrodes and the coating solution may be made in any convenient manner, such as in the actual coating bath or holding tank of the coating solution as it is being used, as well as in a sample portion or stream of the coating solution removed from the main body of the coating solution for purposes of making the measurements. In general, the specific manner in which these measurements are made will depend upon the particular type of operation involved, i.e., whether it is a spray, immersion, roller coating or the like application, as well as on the particular type of equipment which is used.

The fluoride sensitive electrode of the potentiometric type which is used, has, as its ion-sensitive element, a membrane of a substantially imporous, solid, ionic fluoride which is substantially insoluble in the solution. Electrodes of this type are described in US. Pat. 3,431,182 which issued Mar. 4, 1969. As described in this patent, the ion-sensitive crystalline fluoride of which the imporous membrane of the electrode is formed is selected from lead fluoride and the trifluoride of bismuth, scandium, yttrium, and the lanthanide series of rare earth metals. Of these, the preferred electrodes for use in the method of the present invention are those in which the membrane is formed of monocrystalline lanthanum trifluoride, although electrodes utilizing the other ion-sensitive crystalline fluorides disclosed in this patent are also suitable.

The reference electrode which is used in the present method may be any of the commonly used reference electrodes, as are well known to those in the art. Such electrodes include the standard calomel electrode, the standard Ag-AgCl electrode and the like. Both the reference electrode and the fluoride-sensitive electrode are electrically connected to the respective inputs of a suitable electrometric device for measuring the potential between the two electrodes when they are in contact with the coating solutions. Suitable electrometric devices which may be used include various volt meters, particularly the high-input impedience type, expanded millivolt scale pH meters, and the like.

In addition to measuring the potential-between the reference electrode and the fluoride-sensitive electrode in the actual conversion coating solution which is used, a similar measurement of the potential between these two electrodes is also taken in a second or control aqueous conversion coating solution. This second aqueous conversion coating solution is one in which the composition, i.e., the concentration of each of the components of the solution is known, and where it is also known what weight of conversion coating will be formed by this solution. After measuring the potential between the two electrodes in this second conversion coating solution, the difference between the two potential measurements is determined and, thereafter, in the operation of the coating process, this difference in potential is maintained at a value which will consistently produce the desired weight of conversion coating on the metal surface being treated.

The maintenance of the desired difference in potential is effected by the selected addition of one or more of the various solution components to the aqueous conversion coating solution which is being used to treat the metal surface, i.e., the first conversion coating solution. Thus,"

as changes in the value of this difference between the potentials of the two solutions take place, additions of fluoride and/or the other solution components, such as the chromate or phosphate, or additions of Al, e.g. by processing metal, may be made to the coating solution so as to maintain the value of the potential difference at the level which will consistently provide the desired coating weight on the metal surface treated.

In determining the value of the difference in potential between these twosolutions which is to be maintained, a plot or graph is first developed of the coating weights produced versus potential difference. Thereafter, by reference to this plot, it can be determined what the potential difference should be to produce the coating weight that is desired. In preparing such a graph, a series of conversion coating solutions is prepared, containing varying, but known amounts of the solution components. For ex ample, a series of chromate-phosphate coating solutions, each containing different amounts of the fluoride accelerating ions, would be formulated. Each of these standard solutions would then be used to coat the metal surface and then the coating weight produced by each would be determined. These determinations may be carried out using conventional and well known techniques, such as by weighing the coated samples, stripping the coating from the sample with nitric acid or a molten salt bath, such as molten caustic soda, and then reweighing the metal sample. A measurement of the potential of each of these standard coating solutions, using the reference electrode and fluoride-sensitive electrode as described above is also made. Thereafter, one of the standard coating solutions is taken as the zero point and the difference between the electrical potential of this solution and each of the other standard solutions is then determined and these differences are ploted against the coating-weight which is produced by each of the solutions. This plot will then be used, as indicated above, to determine the potential difference which should be maintained in order to consistently obtain the desired coating weight on the metal surface being treated.

In preparing the standard solutions, the concentration of solution components used should be chosen such that the coating weights produced by these solutions will bracket the coating weight which it is desired to obtain on the actual processing line. Thus, for example, if it is desired that the coating line will produce a coating weight on the metal surface treated of 20 milligrams per square foot, the standard conversion coating solutions prepared will be selected so that they will produce coating weights of from about 10 milligrams per square foot to about 30 milligrams per square foot. The particular standard which is then used as'the'zero point for the preparation of the potential difference versus coating weight curve will then be used as the control or second conversion coating solution from which the difference in potential with the operating coating solution is determined. Thus, for example,where the solution giving thecoating weight of 30 milligrams. per square foot is chosen for the zero point on the curve, and it is desired to treat the metal surfaces to obtain a coating weight of 20 milligramsper square foot, the diflerence'in potential between this solution and one which will give 'a coating weight of 20 milligrams per square foot isread olf'of the curve and this difference in potential is'then maintained between the con trol solution and theoperating conversion coating bath. By operating in'this manner, it-is not necessary to effect repeated standardizations of the measuring or control equipment since the control of coating weight is-not based on the actual potential of the coating solution, in millivolts, but rather on' the difference in electrical potential between this solution and the control. For example, if the control coating solution which produces 30 milligrams perfsquare foot of coating weight has "an actual potential reading of '25 millivolts on one day and "an actual potential reading of 30 millivolts on another day, due to shifts or changes in the measuring apparatus, and it is known that a potential difference of millivolts between the control solution and the operating solution is-necessary to produce the desired-coating weight of 20 milligrams per square foot, restandardization ofv the equipment on the second day so that the control reading is 25 millivolts is unnecessary, since the operating solution potential will also have increased by 5 millivolts on the second day. Thus, it is immaterial, from the standpoint of the operation of the present control method, whether the control solution and the operating solution have a potential of 25 millivolts and millivolets respectively, i.e., a potential difference of 10 millivolts, or a potential of 30 millivolts and millivolts respectively, since the potential difference between the two is still the same, i.e. 1-0 millivolts.

It is to be noted, however, that in making the potential determination of the control solution or the second solution, and the operating solution or the first solution, that the conditions under which these measurements are made are similar, particularly with respect to the solution temperatures. In general, it has been found to be preferable to operate the coating processes at substantially room temperature, or somewhat above, and that the temperature of boththe operating solution and the control solution be substantially the same.

It is to be appreciated that the measurement, determinations, and controls effected in the present method may be carried out either manually or by means of various automatic sensing and control equipment, as is familiar to those in the art. Such equipment may include various measuring and sensing devices, with suitable switches and relays whereby the potential of the operating coating bath may be continuously monitored so that upon a deviation from the desired difference in potential between this bath and the control bath, suitable pumps are activated to supply the desired solution components to the operating solution to reestablish the desired difference in potential. This automatic control equipment may also include similar equipment for monitoring the concentrations of the other components in the coating solution, such as by electrical conductivity, pH, and the like. Typical of equipment of this type which may suitably be used for these purposes is that set forth in US. 'Pat. 3,515,094 issued June 2, 1970 and US. Pat. 3,312,189, issued Apr. 4, 1967. It is believed that those skilled in the art will readily be able to determine other suitable equipment of this or similar type which may also be used.

' By use of the control method of the present invention, it is found that once the initial plot is made of the potential differences versus the coating weights produced by the standard solutions, the time consuming determinaf tions of coating weight during the operation of the process are no longer necessary. Additionally, it is found that other than thermostating of the samples whose potential is beingmeasured, other sample preparation is'not required.. Additionally, the measurements obtained in the control of the solutions give an-immediate indication of whether the aqueous conversion coating bath contains too much or too little fluoride and permits the process operator to adjust the replenishing schedule of-the oper ating bath if .necessary. It is further found that these advantages are obtained without the need for repeated standardizations of the measuring equipment which is used.

harder that those skilled in the art may better under: stand the present invention and the manner in which it may be practiced, the following specific example is given. In this-example, unless otherwise indicated, parts and percent are by weight and temperatures are in degrees centigrade. V r

. EXAMPLE A standard aqueous chromating bath was prepared containing the following components:

. 'Percent by weight CrO 0.5 P0 1.59 F 0.08

When applied to an aluminum sheet, by spraying for 5 seconds, this bath produced a coating weight of 18 milligrams/ftr.

From this standard bath an artificially aged bath was prepared having the following makeup:

Percent by weight CI'O3 0.5 P0 3.7 Al+ 0.43 F 0.94 Cr+ 0.41

In this bath the 0.94% F was the total F contained, of

which about 0.9% is tied up with the Al+ giving a free Coating wt. Percent (sample) Total F- in Potential Potential Potential mcremilh- (stand- (sample) difference ments grams] ard) in in standard added it. millivolts millivolts sample Using these values the graph, as shown in FIG. 1, of coating weight vs. potential difierence (Amv.) was prepared. This graph and the standard bath described above were then used, with fluoride-sensitive electrode and system described, to control an actual operating bath.

The operating bath used had the following composition:

Percent by weight cro, 0.5 Po 4.01 F 0.84

Al 0.36 Cr+ 0.25

' Potential Potential (operating Predicted Actual (standard) bath) in Potential coating wt. coating wt. in millivolts millivolts difference in rug/it. in mgJft.

It was found that by maintaining the potential difierence between the standard and the operating. bath at about -7.5, a coating weight of about 18 milligrams/ft. was consistently produced. This was done by additions of HF or Al as were necessary to lower or raise the potential difference.

While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention as it is realized that changes therewithin are posible and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially-the same or equivalent manner,.it beinglintended .to coverthe invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A method for controlling the weight of conversion coatings produced upon a metal surface by a fluorideaccelerated aqueous conversion coating solution which comprises contacting a reference electrode and a fluoridesensitive electrode of the potentiometric type with a first fluoride-accelerated aqueous conversion coating solution, measuring the total potential between said two electrodes in said solution, contacting said two electrodes with a second fluoride-accelerated aqueous conversion. coating solution, the composition of which" and the weight of conversion coating formed by which 'is known, measuring the total potential between said two electrodes in said second solution, determiningthe difference between saidtwo potentials, and maintaining this difference at a value which will consistently produce the desired weight of conversion coating by said first solution by the selected addition, of solution components to said first conversion coating solution. a

2. The method asclaimed in claim 1 where the aqueous conversion coating solution is an aqueous chromating solution.

3. The method as claimed in claim 2 wherein the aque ous conversion coating solution contains both .hexavalent chromium and phosphate ions.

References Cited V UNITED STATESPATENTS 2,928,763 3/1960 Russelletal 148-6.16 3,312,189 4/1967 McVey 118 7 3,370,992 2/1968 Ilenda 61311. 148'-6 .16 3,431,182 3/1969 Fraut 204-11" 3,442,782 5/1969 Shilleret al 204 19sM 3,515,09 6/1970 McVey 118- 5 TA-HSUNG TUNG, Primary Examiner US. Cl. X.R. 

